That's also an all-time high, as well as a 0.9-percent increase over the same period in 2014, and a 22-percent increase over the "base index score" set in the fourth quarter of 2007, according to Wards.

The industry trade journal also notes that light truck and SUV sales have increased in recent months.

But it claims the smaller, lighter segments within each vehicle type have gained market share at the expense of larger, heavier full-size vehicles.